Field of the Invention:
The invention relates to a method of metering dampening solution when printing with a printing form for offset printing. In order to achieve a desired printing result when printing with a wet offset printing machine, it has been known to control or regulate the proportion of dampening solution in an emulsion of printing ink and dampening solution to be applied to a printing form. The actual portion or share of dampening solution can be determined with suitable detectors which are directed towards the surface of the printing form. The portion can be determined indirectly by evaluating signals from an image recording device or a densitometer, which are directed toward locations on the printing material which are free of a printed image, the extent of scumming or smearing that occurs being a participant therein. The composition of the emulsion must be set to an optimum value before printing is begun and during the production of first prints, otherwise prints which are not of suitable quality will be produced. The composition of the emulsion is disrupted if printing is interrupted. In order to obtain the desired composition of the emulsion as quickly as possible, it has become known to continue to dampen the printing form or plate for a predefined number of revolutions of the form or plate cylinder after printing has been interrupted. Furthermore, it has become known to set the supply of dampening solution operating again for a predefined number of revolutions of the plate cylinder before printing is continued. Post-dampening after an interruption may last for thirty-two revolutions, for example, whereas predampening before continuing to print may be performed for six revolutions of the plate cylinder. Premoistening may take place initially with a maximum supply rate and be continued with a supply rate that is suitable for continuous printing. In the case of rotary printing machines, premoistening and after or post-moistening may take place on the surface of the printing form or plate or on the surfaces of inking rollers which can be brought into contact with the surface of the printing form or plate. In conventional inking units, bridge rollers are provided, which produce a connection between rollers feeding dampening solution and printing ink (published German Patent Document DE 195 06 639 C2).
The procedure described hereinabove is implemented by control devices, which operate in accordance with a program. A printing machine operator, however, also has the possibility of changing the predampening and after or post-dampening times by manual entry, how the entered numbers of revolutions of the plate cylinder are optimally adapted or matched to the properties of the printing material, the printing image, the material of the printing form or plate, the transfer properties of a transfer cylinder and the properties of ink and dampening solution being determined by the experience of the operator. The operator cannot take into account the frequency and the duration of the interruption, because they are unable to be predicted by him or her.